The present invention relates to a joint structure for synthetic resin structural members such as fiber reinforced plastics (FRP) members which is effective for application of synthetic resin structural members instead of conventional timber members and more specifically effective for application of FRP members as a main framework in a cooling tower so as to increase the corrosion resistance and to simplify the operational control.
Many structural members of synthetic resin have been used instead of timber and metal. However, such synthetic resin members have scarcely been used in circumstances where they are subject to a heavy load.
For instance, in the field of cooling towers for cooling warm water by direct contact with a large quantity of surrounding air, synthetic resin members have been used only partially for small cooling towers for air-conditioning systems or the like.
Generally, large cooling towers, as are used in various chemical industries, include a main or load-bearing framework of timber or steel. Though being typically composed of Douglas fir timber which has been subjected to a pressure-preservative treatment or steel members plated with zinc in a hot dip galvanizing process, the main framework still tends to be easily corroded since it is within the tower which has an atmosphere of extremely high temperature and of extremely high humidity.
A framework composed of timber or steel members typically begins to be corroded after about ten or seven to eight years, respectively, at and around the upper portion of the framework adjacent to a distribution system, and this necessitates repair of the tower.
As the corrosion spreads, the corroded pieces of timber or rust particles disperse into the cooled water, which may result in deterioration of the performance of the cooling tower, such as by clogging of the strainer in the water-cooling system or an increase in the load to which the pump is subjected.
It may be therefore contemplated that the main framework is fabricated from synthetic resin, especially FRP, which exhibits high corrosive resistance in corrosion environment. The main framework cannot be fabricated from single FRP member and is an assembly of columns and horizontal and cross beams so that a structure for securely joining them together must be developed.
To this end, various joint structures have been developed by some of the inventors or the like. For instance, a joint structure for joining hollow FRP members rectangular in cross section was developed as disclosed in Japanese Patent 1st Publication No. 2-245506 (its patent family: U.S. Pat. No. 5054197 and European Patent Laid Open Specification No. 0388222 A2) in which, as shown in FIGS. 1a and 1b, for example two hollow FRP members 1 and 1a rectangular in cross section are joined together by a joint 2 in the form of a hollow FRP member rectangular in cross section, surrounding the members 1 and 1a and having predrilled rivet holes on their side walls. More specifically, the members 1 and 1a to be joined together are fitted through spacers 4 into the joint 2 such that opposed ends of the members 1 and 1a abut against each other. Then, at a site, rivet holes are drilled through side walls of the members 1 and 1a so as to align with the predrilled rivet holes on the corresponding side walls of the joint 2 and blind rivets 3 are driven into these rivet holes. Such operation is repeated to complete all the riveting, whereby the members 1 and 1a are joined together.
In this manner, whenever one blind rivet 3 is to be driven into the members 1 and 1a, a corresponding rivet hole is drilled through the members 1 and 1a and such drilling and riveting are repeated to complete the joining of the members 1 and 1a. Therefore, even when a great number of blind rivets 3 are to be used, rivet holes can be formed with even drilling accuracy so that load can be evenly distributed over the whole blind rivets 3, whereby a required joint strength is obtained.
However, such drilling and riveting operations of the FRP members 1 and 1a through the joint 2 and the blind rivets 3 at a site themselves are very hard and cumbersome as compared with a case where a main framework is constructed from conventional timber structural members. The number of the assembling steps of the former is disadvantageously increased by 10 to 20% as compared with that of the latter.
In order to overcome the problem, it may be contemplated that instead of the blind rivets 3, bolts are used each of which has strength greater than that of a blind rivet. When the joint and the FRP structural members are respectively predrilled for that purpose or for bolting, the diameter of the drilled bolt hole must be greater than that of a bolt to be used by 2 to 3 mm and therefore the load cannot be evenly distributed over the whole bolts because of the drilling and bolting tolerances or inaccuracies. This means that even in use of a plurality of bolts, the whole allowable load does not increase in proportion to the number of bolts (See the broken line in FIG. 4).
It may be suggested that improvement of the drilling accuracy for even distribution of the load over the whole bolts is attained by on-site drilling of a FRP structural member for alignment with the predrilled bolt holes on a joint. However, in fact, it is next to impossible to drill bolt holes diametrically through a FRP structural member in the form of hollow pipe with a required degree of drilling accuracy at a site.
In joining of FRP structural members using a plurality of bolts, however the bolt holes are predrilled with a high degree of drilling accuracy, the pitch of the bolt holes is preferably four times or more as long as the diameter d of the bolts as shown in FIG. 2b so as to ensure a sufficient degree of bearing strength. As a result, portions of the structural members to be used for the joining becomes longer so that a joint to be used becomes also larger and therefore the bolting operation at a site becomes hard.
In order to overcome the above and other problems encountered in the conventional joint structure for joining structural members, the present invention was made to provide a joint structure for securely joining synthetic resin structural members with a plurality of bolts in a simple manner.